Wireless communication systems are continually striving to increase the data bandwidth so that information can quickly be exchanged between devices coupled to the communication system. Some of the parameters that limit the data bandwidth available to devices include the spectral bandwidth allocated to the devices and the quality of the channel linking the devices.
Wireless communication systems compensate for the various constraints on data bandwidth using a variety of techniques. A wireless communication system may incorporate multiple encoding techniques, and may select an encoding technique based on a data rate supported by a channel. In such a system, the communicating devices may negotiate a data rate based on the capabilities of the channel. Such a communication system may be advantageous for multiple point to point links, but may be less than ideal in a distributed broadcast system where a single transmitter provides substantially the same data to multiple receivers.
Wireless communication systems may incorporate hierarchical modulation, also referred to as layered modulation, where multiple data streams are simultaneously transmitted across a hierarchy of data layers. The multiple data streams can include a base layer that is a robust communication link capable of successful reception in nearly all receiver operating conditions. The multiple data streams can also include an enhancement layer that is broadcast at a data rate that is lower, the same, or higher than the data rate of the base layer. The communications over the enhancement layer may require a higher signal quality at the receiver compared to the base layer. Therefore, the enhancement layer may be more sensitive to variations in the quality of the channel.
The receiver is typically ensured the ability to communicate at the base level, and can typically demodulate data on the base layer. In channel conditions sufficient to support the enhancement layer, the receiver is also able to demodulate additional data modulated on the enhancement layer to provide a higher quality of service or to provide additional data bandwidth.
The use of layered modulation signals substantially complicates the operation of the transmitter and receiver. Some data streams may have unrelated information on each of the base layer and enhancement layer. Because of the relative independence of the two layers, one layer, for example the enhancement layer, may complete a transmission or otherwise have no additional information to transmit while the base layer continues to supply information. The transmitter may not have the ability to selectively activate and deactivate layered modulation at the same rate that is needed to support intermittent operation of the enhancement layer. Alternatively, incorporating unlimited ability to select between layered or non-layered operation may be undesirable or unfeasible due to hardware constraints, cost, or established standards.
The intermittent lack of information for the various layers of a layered modulation signal creates potential problems at the transmitter and the receiver. The transmitter needs to have the ability to operate when there is information on less than all layers of a layered modulation signal. Additionally, the receiver needs to be able to discern when less than all layers of a layered modulation signal carry information. However, any potential solution must not overly burden the processing capabilities of the transmitter or the receiver.